While current histopathological criteria permit classifying the majority of melanocytic tumors as either benign nevi or melanoma, well-documented uncertainty exists in a significant number of cases. Misclassification results in inappropriate over- or under-treatment of patients. Our recent work using comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH) on primary tumors has shown that the pattern of genomic aberrations differs significantly between clearly identifiable melanoma and benign nevi. The vast majority of primary melanomas have multiple chromosomal aberrations, whereas the vast majority of nevi do not have any. The few benign nevi that do have aberrations typically have a very restricted set, which does not occur in melanoma. This project will determine if a similar genomic analysis would help distinguish between benign and malignant lesions that are ambiguous by current histopathological criteria, and if those morphological criteria can be improved. In this project we will use two separate cohorts of histologically ambiguous melanocytic tumors that have extensive follow-up in order to systematically screen for genomic and histopathological markers that can predict outcome. The first cohort will serve as a training set and the second as a test set for validation. Cases will be contributed by a panel of internationally recognized pathologists with great expertise in melanocytic tumors. The genomic analysis will take advantage of array-based CGH. This technology has recently been developed in our laboratories and provides a resolution of approximately 1 megabase across the human genome. This technique can be performed with small amounts of DNA extracted from routinely fixed archival tissue from primary tumors. First, we will use array CGH on the training set to screen for genomic aberrations that distinguish metastasizing and non-metastasizing cases and the expert panel will use these same tumors to develop improved morphological classification criteria. The genetic and morphological criteria will be built into classification rules using the training set. The vies that work best on the training set will be validated on the independent set of tumors. Finally, we will implement the genomic rule in form of hybridization probes for a few specific loci and develop a FISH-based test. The long-term goals of this project are to find genetic and morphological criteria that can classify melanocytic tumors that are low ambiguous and to develop a prototype clinical test for this purpose. Such a test would be of significant clinical relevance, as it would help to identify patients who need additional treatment, and prevent others from getting inappropriately aggressive treatment. In addition, this project will provide a detailed view of the aberrations found in melanocytic tumors, their prevalence and prognostic relevance.